Calendar of Physics Talks Vienna

K3 string theories, symmetries and wall crossing

Speaker:

Roberto Volpato (Padua University)

Abstract:

I will consider a large class of four dimensional N=4 string models obtained from compactifications of type II string theory on $K3\times T^2$ and orbifolds thereof. I will show that the multiplicities of 1/4 BPS states in such models can be determined (almost) uniquely by imposing some simple consistency condition. The main ingredient in this derivation is a careful analysis of the wall crossing phenomenon. These results lead to a better understanding of the action of discrete groups of symmetries on the BPS spectrum of K3 string models and might shed some light on the mysterious Umbral moonshine conjectures. This talk is based on joint work with Natalie Paquette and Max Zimet.

The moduli spaces of stable vector bundles on surfaces have a well-understood structure—generically
smooth of the expected dimension, and irreducible—for large values of c2; and they are empty for c2 less than
the Bogomolov-Gieseker bound.We’ll discuss joint work with Nicole Mestrano about understanding the structure
for all values of c2 in between, for generic hypersurfaces of low degree in P3.

Higher-spin fields can play an important role in effective field theories and increasingly
are of interest due to their appearance in recent studies of holography. Understanding their
interactions is however often involved and searching for alternative formulations could be
very useful. In this talk we will consider one such formulation, namely twistors. After giving
a general introduction to the use of twistors in describing the dynamics of massless field theories, we will focus on the construction of off-shell actions directly in twistor space including a recent higher-spin generalisation of self-dual Weyl gravity. We will show how this formulation produces the expected flat space-time spectrum and linearised symmetries.
In analogy with the known embedding of Einstein gravity inside Weyl gravity, we identify
a ghost free sub-sector of the conformal higher-spin theory and . . .

More than half of all medical drugs target membrane proteins; however, the complex intracellular protein interactions are mostly unknown. Therefore, we are developing a method to study protein-protein-interaction with membrane proteins in a living cell. As model organism we have chosen the T-cell with its T-cell receptor TCR and the related membrane protein CD4. For this method a substrate is required with nanopatterns of streptavidin and fibronectin, catching the cell and reallocate CD4. While for patterns with dimensions in the realms of micrometer contact printing with Polydimethylsiloxan (PDMS) is well established, this is not suitable for nano features. Thus we had to test different materials and methods before we finally were able to show images with total internal reflection fluorescence microscopy (TIRFM) of nanopatterned membrane proteins in vivo.

Strong Cosmic Censorship and the initial singularity in Bianchi spacetimes

Speaker:

Katharina Radermacher (Stockholm)

Abstract:

For given initial data to Einstein's field equations, one can find a spacetime solving these equations, and one can do so in a unique way (up to isometries) if one assumes the spacetime to be maximal globally hyperbolic. Both statements were proven by Choquet-Bruhat and Geroch in the 1950s and 60s. When dropping the additional condition of global hyperbolicity, it is an open question whether one can extend this spacetime, possibly in a non-unique way. Strong Cosmic Censorship conjectures that no such extension exists, at least not for generic initial data. In my talk, I focus on spacetimes where the initial data is symmetric under the action of a three-dimensional Lie group (a so-called Bianchi spacetime) and the stress-energy tensor is that of vacuum or a perfect fluid. I present results proving the Strong Cosmic Censorship conjecture for orthogonal Bianchi class B spacetimes and . . .

Phillip Griffiths (Institute for Advanced Studies, Princeton and University of Miami)

Abstract:

The bundles that arise in Hodge theory tend to have signs (say positive). The singularities that arise have the properties: they should be thought of like wave-front-sets in cotangent bundles; they are mild; and they increase the positivity. These properties with some applications to algebraic geometry will be discussed.

im Rahmen des Lunchseminars der Gravitationsphysik: Black hole entropy S is one of the most fascinating issues in contemporary physics, as one
does not yet strictly know what are the degrees of freedom at the fundamental microlevel,
nor where are they located precisely. In addition, extremal black holes, in contrast to non-
extremal ones, present a conundrum, as there are two mutually inconsistent results for the
entropy of extremal black holes. There is the usual Bekenstein-Hawking S = A/4 value,
where A is the horizon area, obtained from string theory and other methods, and there is the
prescription S = 0 obtained from Euclidean arguments. In order to better understand black
hole entropy in its generality, we exploit a matter based framework and use a thermodynamic
approach for an electrically charged thin shell. We find the entropy function for such a system. We then take . . .

im Rahmen des Teilchenphysikseminars: In the last few years a series of hints of deviations from the Standard
Model in B-meson decays have been reported by experiments.
I review these hints, focusing in particular on the tests of Lepton
Flavor Universality, addressing both Standard Model uncertainties
and discussing possible New Physics interpretations.

Efficient interaction of light with quantum emitters is crucial for most applications in nano and quantum technologies. In the last decade, great progresses in the field have been possible by exploiting hybrid interfaces. In this context, we provide experimental proofs of the efficient coupling of single organic molecules to confined electromagnetic modes in photonic devices.
Dibenzoterrylene molecules in anthracene crystals 8DBT:Ac) are particularly suitable quantum systems for this task, due to outstanding photo physical properties in crystals as thin as few tens of nanometers: long-term photo stability, lifetime-limited emission in the zero phonon line at cryogenic temperatures, operating wavelength in the near-infrared at 780 nm. Building on the advantages of all-solid-state systems, we demonstrate that our platform allows effective single photon sources specifically coupled to confined radiation modes and opens the pathways to the realization of localized optical nonlinearities at the single-photon level.
A versatile planar optical antenna-like structure is demonstrated, which strongly directs the radiation of the molecule into a narrow pattern (20° wide semicone, thus enabling considerably effective mode-matching with an external Gaussian beam. We also report on our results about fluorescence coupling to a single-mode dielectric waveguide and discuss the integration of single quantum emitters into hybrid dielectric-plasmonic devices, with respect to the realization of optical transistors in the low light regime. Competitive collection efficiencies, free from interconnection losses, candidate the system for scalable approaches to on-chip quantum computation. Eventually, we report on current efforts to integrate molecular crystals in polymer made architectures.